The invention relates to a system for removing impurities from low-boiling point gases, primarily by low-temperature scrubbing.
An enriched hydrogen gas is conventionally obtained by the purification of coke oven gas on the one hand, or cracking gases on the other hand, the latter being derived from the thermal cracking of natural gas or other hydrocarbon mixtures. These gaseous mixtures must be treated to remove undesired components from the hydrogen. There are a variety of techniques which can be used for this purpose. Thus carbon dioxide, water and higher hydrocarbons are predominantly removed by condensation in regenerators, whereas carbon dioxide and hydrogen sulfide impurities are frequently removed by a methanol scrub and carbon monoxide impurity is chemically converted during which further hydrogen is formed, the final step being one of methanization.
When hydrogen is to be used for ammonium synthesis, the last purification stage may be liquid nitrogen scrub thereby permitting such low boiling-point impurities as methane and carbon monoxide to be removed from the gas. A nitrogen scrub will always, however, entrain an appreciable amount of nitrogen gas with the purified hydrogen. Whereas such entrainment is well suited for preparing hydrogen for ammonium synthesis, because nitrogen addition is necessary to obtain a stoichiometric mixture of 3H.sub.2 + N.sub.2, such N.sub.2 entrainment is not suited for other purposes when the hydrogen must be nitrogen-free. In the latter case, the final purification stage often involves scrubbing with liquid methane, whereby hydrogen purities up to 99% may be achieved. However, further purification of hydrogen beyond 99% by methods heretofore known is extraordinarily costly.